A super absorbent polymer (SAP) is a synthetic polymeric material capable of absorbing water five hundred up to one thousand times the mass of its own. The SAP is called by different names according to the developers, for example, a super absorbency material (SAM), an absorbent gel material (AGM), and the like.
Commercial productions of the SAP began for use in feminine hygiene napkins, nowadays, the use of SAP may be found in a very wide range of applications, including water combination soils for gardening, water-stop materials for engineering and construction, sheets for raising seeding, freshness-keeping agents in the food distribution field, and materials for sweating rooms, as well as personal hygiene products, such as disposable baby diapers.
Known preparation methods for SAP include an inverse suspension polymerization method or an aqueous solution polymerization method.
The method using inverse suspension polymerization is disclosed in, for example, JP S-56-161408, JP S-57-158209 and JP S-57-198714.
The method using aqueous solution polymerization includes several methods: a thermal polymerization method including polymerization of a hydrated gel polymer by shearing and freezing in a kneading machine provided with several spindles, and a photopolymerization method using UV radiations on a high-concentration aqueous solution on a conveyer belt to perform both polymerization and drying at once.
A hydrogel polymer obtained through the polymerization reaction as described above is generally pulverized through a drying process, and then commercially available as a powdery product.
Here, during cutting, milling and pulverization of the dried polymer, a fine powder having a particle size of about 150 μm or less may occur.
When super absorbent polymer particles prepared by including the fine powder is applied to products, they may exhibit physical properties that are shifted or lowered before use, and thus, the fine powder is considered to be undesirable in hygiene articles including infant diapers and urinary incontinence equipment for adults.
Thus, a reassembling process in which the fine powder is excluded so as not to include the fine powder in a final resin product or the fine powder is aggregated to have a normal particle size. In many cases, the reassembled polymer is crushed into a fine powder again due to a low aggregation strength.
Therefore, there is still a need for a manufacturing method of a super absorbent polymer capable of having a high fine aggregation strength to reduce a fine occurrence rate of the reassembled polymer.